Field of the Invention
The present invention relates to an image forming apparatus, for example, a copying machine or printer, which is provided with a function for forming an image on a recording material, such as a sheet.
Description of the Related Art
Image forming apparatuses based on an electrophotographic system include an image forming apparatus based on an intermediate transfer system which outputs an image by primary transfer of a toner image from a photosensitive body to an intermediate transfer belt, and then secondary transfer of the toner image onto a recording material. An intermediate transfer belt of an endless belt shape is widely used as an intermediate transfer belt. Broadly speaking, methods for cleaning remaining toner on an intermediate transfer belt in an intermediate transfer method are: a blade cleaning method, an electrostatic cleaning method and a hybrid method combining these.
The blade cleaning method is a method in which, as disclosed in Japanese Patent Application Publication No. 2009-288481, a cleaning blade is placed in contact with the intermediate transfer belt, and remaining toner on the intermediate transfer belt is physically scraped away by this cleaning blade. This cleaning method can be expected to provide good cleaning properties at low cost, but is liable to the effects of wear of the blade and unevenness of the surface of the intermediate transfer belt due to enduring use, and hence there is a concern that good cleaning performance cannot be maintained for a long period of time.
In the electrostatic cleaning method, as disclosed in Japanese Patent Application Publication No. 2009-205012, remaining toner is charged to the opposite polarity of the charged state during development, by a charging unit that applies a voltage to the remaining toner. Thereupon, the remaining toner which has been charged to the opposite polarity is transferred from the intermediate transfer belt to the photosensitive body in the next primary transfer step, and is collected by a cleaning unit that cleans the photosensitive body. Therefore, this method is known as a simultaneous transfer and cleaning method. The electrostatic cleaning method has an advantage in not being liable to the effects of unevenness in the surface of the intermediate transfer belt, but the following concerns arise when processing a large amount of remaining toner on the intermediate transfer belt, such as after dealing with a paper jam or after calibration. More specifically, when processing a large amount of remaining toner on the intermediate transfer belt, a large amount of toner adheres to the charging unit and this needs to be cleaned in order to maintain the cleaning performance. The cleaning by the charging unit in the electrostatic cleaning method involves ejecting (moving) the adhering toner by applying a bias of the same polarity as the toner, from the charging unit, and then collecting the ejected toner on the photosensitive body.
However, in collecting the ejected toner, since the charging polarity of the toner immediately after ejection is opposite to that of the primary transfer bias, then in a primary transfer portion the ejected toner cannot be collected on the photosensitive body immediately after being ejected. Therefore, the intermediate transfer belt must be rotated further and the ejected toner must be charged again to the same polarity as the primary transfer bias by the charging unit. Consequently, in the cleaning of the intermediate transfer belt after a jam or calibration, time is required to rotate the intermediate transfer belt that is used in the ejection step, and if this is long, then multiple rotations of the intermediate transfer belt may be necessary.
The hybrid cleaning method disclosed in Japanese Patent Application Publication No. 2000-131920 is a cleaning method of the following kind. Firstly, remaining toner on the intermediate transfer belt is removed generally by a cleaning blade situated to the downstream side of the secondary transfer portion in terms of the direction of rotation of the intermediate transfer belt. The remaining toner not scraped away by the cleaning blade is charged by the charging unit which is disposed to the downstream side of the cleaning blade in terms of the direction of rotation of the intermediate transfer belt, whereby simultaneous transfer and cleaning is performed onto the photosensitive body. In this hybrid method, since a large amount of remaining toner is not supplied to the charging unit, then there is no adherence of toner to the charging unit, even under conditions that give rise to a large amount of remaining toner, such as after a jam or calibration, etc. Therefore, surplus time for rotating the intermediate transfer belt to remove this toner is not necessary. Therefore, the hybrid cleaning method can achieve the smallest processing time (downtime) of the abovementioned three cleaning methods, and can achieve good cleaning performance over a long time.
However, in a hybrid cleaning method such as that described above, there is a concern in that problems such as the following arise when the size of the apparatus is reduced and/or the printing speed is increased. In order to achieve good cleaning performance with the hybrid method, it is necessary to charge the toner not scraped away by the cleaning blade, uniformly, to an opposite polarity. Therefore, the charging unit needs to generate an electric discharge that is at least capable of reversing the amount of charge on the toner that has not been scraped away. On the other hand, the charging unit also charges the surface of the intermediate transfer belt, as well as the toner, and therefore the amount of electric discharge needs to be limited in such a manner that the surface potential of the intermediate transfer belt after passing the charging unit does not affect the next primary transfer operation.